This invention relates to battery charging. In particular the invention relates to methods and apparatus for fast battery charging which provide a charge cycle during which a battery is periodically discharged. The invention has particular application in the rapid charging of lead-acid batteries.
Charging a battery involves passing electrical current through the battery from a suitable direct current electrical power supply. The rate of charge depends upon the magnitude of the charging current. In theory it is possible to reduce charging time by using a higher charging current. In practice, however, there is a limit to the charging current that can be used. All batteries have some internal resistance. Power dissipated as the charging current passes through this internal resistance heats the battery. The heat generated as a battery is charged interferes with the battery""s ability to acquire a full charge and, in an extreme case, can damage the battery itself.
Because the charging rate is limited it can take a long time to charge a battery to its capacity. In some cases, battery charging times as long as 16 hours are standard. The charging time for a particular battery depends upon the capacity and construction of the specific battery at issue.
Another problem with current battery chargers is that they are not always designed in a way which optimizes the service lives of batteries being charged. Some chargers provide an excessive charging current in order to provide reduced charging times.
Lead-Acid batteries are still the most practical type of battery for many heavy-duty applications such as engine-starting, powering electric vehicles, such as forklifts and the like. It is well known that Lead-Acid batteries should be charged within certain general parameters. It is generally considered that a Lead-Acid battery should never be charged to its full capacity at a rate greater than 10% to 15% of the battery""s capacity. Faster charge increase battery temperature and may damage battery causing harmful over-charge and reduce its life. Larger charging currents may be applied for short periods when the battery is nearly flat to xe2x80x9cboostxe2x80x9d the battery. A typical multi-stage charger for Lead-Acid batteries applies three charge states. For a first period the charger passes a constant charging current through the battery. In a second period the charger applies a reduced topping charge. Finally the charger applies a float-charge. During the constant-current stage the battery charges to 70% in about five hours. The battery is given the remaining 30% of its maximum charge over about 5 hours by the topping charge. The float-charge which compensates for self-discharge after the battery has been fully charged.
In charging Lead-Acid batteries, it is also important to observe the cell voltage limit. The limit for the cells of a specific battery is related to the conditions under which the battery is charged. A typical voltage limit range is from 2.30 V to 2.45 V.
Some battery chargers have been proposed in which the battery under charge is discharged at various points in the charging cycle. This periodic discharging is said to reduce internal resistance and to reduce consequential heating of the battery. An example of such a charger is described in Pittman et al. U.S. Pat. No. 5,998,968. The Pittman et al. charging cycle applies a 2 millisecond discharge immediately before a 100 millisecond charging pulse. The discharge current is greater than the charging current. This pattern repeats at a frequency of about 10 Hertz. Rider et al. U.S. Pat. No. 5,499,234 is another example of this type of battery charger. The Rider et al. charger periodically discharges a battery with a discharge current which is about equal to the charging current. Ayres et al. U.S. Pat. No. 5,561,360 discloses a battery charger which initially applies a constant charging current. When the battery is partially charged, the Ayres et al. charger begins to periodically discharge the battery.
Patents which show other battery chargers are Samsioe, U.S. Pat. No. 4,179,648; Sethi, U.S. Pat. No. 3,622,857; Jones, U.S. Pat. No. 3,857,087; and, Brown Jr. et al., U.S. Pat. No. 5,617,005.
A common difficulty with battery-powered equipment is the progressive deterioration in reliability after about the first year of service. This phenomenon is mostly due to premature aging of the battery; a reversible capacity loss that is induced by xe2x80x9cmemoryxe2x80x9d. Although fully charged, the battery eventually regresses to the point where it can hold less than half of its original capacity, resulting in unexpected down time. Furthermore, when a battery cannot be fully charged, the battery has a poor ratio of weight to capacity. This is especially significant in electric vehicles.
Some chargers use a temperature sensor, such as a thermistor, to measure the temperature of a battery being charged. The charging current can be reduced when the temperature rises above a threshold. Such temperature sensing is generally inaccurate because of the wide tolerance of the sensing thermistor and its positioning in respect to the cells of the battery.
Some manufacturers claim outrageously short charge times of 15 minutes, or even less, for nickel-cadmium (NiCd) batteries. With a NiCd battery in perfect condition in a temperature-controlled environment it is sometimes possible to charge the NiCd battery in a very short time by providing a very high charge current. In practical applications, with imperfect battery packs, such rapid charge times are almost impossible to achieve.
There is a need to achieve faster charging of batteries. There is a specific need to achieve faster charging of Lead-Acid batteries. There is a particular need for such chargers which are capable of fully charging a battery and do so in a way which does not reduce battery life.
This invention provides methods and apparatus for battery charging that address some of the above-noted deficiencies of prior battery charging technologies. One aspect of the invention provides a method for charging a battery having a capacity C (measured in Ampere hours). The method includes: for a charging period having a duration in the range of about 60 seconds to about 180 seconds, passing an electric current having a magnitude of 0.55xc3x97C to 0.65xc3x97C amperes through the battery; for a discharging period having a duration in the range of about 10 seconds to about 20 seconds allowing the battery to discharge a current having a magnitude in the range of about 0.05xc3x97C amperes to about 0.07xc3x97C amperes through a load; and, repeating these charging and discharging periods in alternating sequence until the battery is charged.
In preferred embodiments the duration of the charging period is in the range of about 100 seconds to about 140 seconds and the duration of the discharging period is in the range of about 10 seconds to about 17 seconds. In preferred embodiments the method includes waiting for a first rest period having a duration of no more than 2% of the duration of the charging period immediately before each charging period. During the first rest period, no current is flowing through the battery. Preferably a second short rest period having a duration of no more than 2% of the duration of the charging period follows each charging period. During the second rest period, no current is flowing through the battery. Preferably the first and second rest periods have durations of less than 200 milliseconds. The invention has specific application to lead-acid batteries and in the preferred embodiments the battery under charge is a lead-acid battery.
The power supply used to deliver charging current to the battery is preferably a constant-voltage power supply.
Another aspect of the invention provides a battery charger for charging a battery having a capacity C. The battery charger comprises: a power supply; a load; an electrically controllable switching circuit configured to connect either the power supply or the load between the terminals of a battery under charge; and, a control circuit comprising a timer, the control circuit configured to, during a charging cycle, operate the electrically controllable switching circuit to, in alternation, connect the power supply between the terminals of a battery under charge for a charging period having a duration of about 60 seconds to about 180 seconds, and connect the load between the terminals of the battery under charge for a discharging period having a duration in the range of about 10 seconds to about 20 seconds. The load has a resistance such that, during the discharge period, the battery discharges at a rate in the range of about 0.05xc3x97C to about 0.07xc3x97C.
In preferred embodiments the power supply is a constant-voltage power supply. Preferably the power supply is configured to supply a charging current having a magnitude in the range of about 0.55xc3x97C to about 0.65xc3x97C during the charging period to the battery under charge.
Some embodiments have a shut-off timer configured to discontinue the charging cycle after a period in the range of 100 minutes to 180 minutes. Most preferably the shut off timer ends the charging cycle in about 2 hours.
Some embodiments have a voltage comparator connected to compare a voltage of a battery under test to a reference voltage. In these embodiments the control circuit is configured to, before initiating the charging cycle, determine if the voltage comparator indicates that the battery voltage is greater than the reference voltage. If so, the control circuit connects the load between the terminals of the battery under charge until the battery voltage is equal to or less than the reference voltage. This ensures that batteries being charged are all started at about the same level of charge.